Conventionally, in an HEV, when an output from an engine is large with respect to motive power required for driving, an electric generator is driven with surplus motive power to charge a secondary battery. On the other hand, when an output from an engine is small, a motor is driven with the electric power from a secondary battery to output supplementary motive power. In this case, a secondary battery is discharged. When a secondary battery is mounted on an HEV or the like, it is necessary to maintain an appropriate operation state by controlling such charging/discharging, etc.
For this purpose, the voltage, current, temperature, and the like of a secondary battery are detected, and the remaining capacity (hereinafter, abbreviated as an “SOC”) of the secondary battery is estimated by computation, whereby an SOC is controlled so as to optimize the fuel consumption efficiency of a vehicle. Furthermore, at this time, in order to allow a power assist based on motor driving during acceleration to be operated and to allow energy to be collected (regenerative braking) during deceleration with good balance, an SOC level is controlled as follows. Generally, in order to set an SOC to be, for example, in a range of 50% to 70%, when the SOC decreases to, for example, 50%, control for excess charging is performed. On the other hand, when the SOC increases to, for example, 70%, control for excess discharging is performed. Thus, it is attempted to approximate the SOC to the center of control.
In order to control the SOC exactly, it is necessary to estimate exactly the SOC of a secondary battery that is being charged/discharged. Examples of such a conventional method for estimating an SOC include the following two kinds of methods.
(1) A charging/discharging current is measured. The value of the current (having a minus sign in the case of charging, and having a plus sign in the case of discharging) is multiplied by a charging efficiency. The multiplied values are accumulated over a certain period of time to calculate an accumulated capacity. Then, an SOC is estimated based on the accumulated capacity.
(2) A plurality of data sets of charging/discharging currents and terminal voltages of a secondary battery corresponding thereto are measured and stored. A primary approximate line (voltage V-current I approximate line) is obtained from the data sets by least squares, and a voltage value (V intercept of a V-I approximate line) corresponding to a current value 0 (zero) is calculated as a no-load voltage (V0). Then, an SOC is estimated based on the no-load voltage V0.
Furthermore, when a secondary battery is charged/discharged, a polarization voltage is generated with respect to a battery electromotive force. More specifically, a voltage increases during charging, whereas a voltage decreases during discharging. This change is called a polarization voltage. In the case of estimating an SOC from a voltage as in the above-mentioned method (2), in the case of estimating an increase and a decrease in a voltage during a predetermined time, and in the case of obtaining electric power that can be input/output during a predetermined time, it is necessary to grasp a polarization voltage exactly.
In general, as a method for estimating a polarization voltage, a primary regression line is obtained from a plurality of current and voltage data, the slope of the line is set to be a polarization resistance (component resistance, reaction resistance, and diffusion resistance), and the polarization resistance is multiplied by a current to obtain a polarization voltage.
However, the above-mentioned two kinds of conventional SOC estimation methods have the following problems.
First, in the case of the SOC estimation method based on an accumulated capacity in the above method (1), a charging efficiency required for accumulating current values depends upon an SOC value, a current value, a temperature, and the like. Therefore, it is difficult to find a charging efficiency suitable for these various kinds of conditions. Furthermore, in the case where a battery is sitting left, a self-discharge amount during that time cannot be calculated. For these reasons and the like, the difference between the true value of an SOC and the estimated value thereof increases with the passage of time. Therefore, in order to eliminate this, it is necessary to perform complete discharging or full charging to initialize the SOC.
However, in the case where a secondary battery is mounted on an HEV, when complete discharging is performed, the secondary battery cannot supply electric power, which becomes a burden on an engine. Therefore, it is necessary to initialize an SOC after stopping a vehicle at a charging site and the like and completely discharging the secondary battery, or after charging the secondary battery over a predetermined period of time until it is fully charged. Thus, in the case of the application to an HEV, it is impossible to perform complete charging/discharging during driving of a vehicle so as to initialize an SOC. Furthermore, periodically performing complete charging/discharging of a secondary battery mounted on an HEV is inconvenient for a user, and also becomes a burden on the user.
Next, in the case of the SOC estimation method based on a no-load voltage in the above method (2), first, a V intercept of a V-I approximate line after large discharging becomes relatively low, and a V intercept of the V-I approximate line after large charging becomes relatively high. Thus, a no-load voltage is varied even at the same SOC, depending upon the past history of a charging/discharging current. This change is caused by a polarization voltage. Accordingly, the no-load voltage that is a V intercept of a V-I approximate line is varied between a charging direction and a discharging direction, due to the factor of a polarization voltage. Because of this, the difference in voltage results in an estimation error of an SOC. Furthermore, a decrease in voltage due to a memory effect and leaving a battery sitting, battery degradation, and the like also cause an estimation error of an SOC.
Furthermore, according to the above-mentioned conventional method for estimating a polarization voltage, when a polarization voltage is obtained from a polarization resistance, a reaction resistance due to the reaction between an active material of a battery and an interface of an electrolyte solution and a diffusion resistance due to the reaction in active materials, between active materials, and in an electrolyte solution, included in a polarization resistance, cannot be estimated sufficiently. Therefore, the accuracy of an estimated polarization voltage is unsatisfactory. Accordingly, it is not practical to use a no-load voltage in the above method (2) for correction, in order to obtain a battery electromotive force for estimating an SOC.